In recent years, along with the rapid expansion of the market for mobile phones and other mobile type electronic devices, magnetoresistive memories (MRAM: magnetoresistive random access memories), ferroelectric memories (FeRAM: ferroelectric random access memories), phase-change type memories (PCRAM: phase-change random access memories), etc. have become the subject of active R&D as next generation nonvolatile memories for taking the place of flash memories. Among these, PCRAM's feature memory cells with simple structures, so are superior in not only manufacturing costs, but also integration degree compared with other memories.
A phase-change material is used for the data recording layer of a PCRAM. Data is recorded utilizing the change in electrical resistance which accompanies a phase change between an amorphous phase (high resistance) and a crystal phase (low resistance) of the phase-change material.
The electrical resistance ratio between the amorphous phase and crystal phase has to be made 102 or more so as to raise the data reading precision.
A phase-change material in the amorphous phase state changes to a crystal phase state by being heated to the crystallization temperature Tc or more. Further, a phase-change material in the crystal phase state changes to the amorphous phase state by being heated to a melting point Tm higher than the crystallization temperature Tc, then rapidly cooling.
For the phase change of the phase-change material between the amorphous phase and crystal phase, the Joule's heat due to application of current or voltage is utilized to heat the material, for example, to the melting point Tm or more to change it to the high resistance state amorphous phase and thereby set the reset state or to heat the material to the crystallization temperature Tc to less than the melting point Tm to change it to the low resistance state crystal phase and thereby set the set state so as to record data.
At the present time, as the PCRAM-use phase-change material, the Ge2Sb2Te5 (GST) which is being used for DVD-RAM's is being broadly studied (for example, see NPLT's 1 and 2).
On the other hand, along with the higher performance of the device, further thermal stability of the phase-change material is sought. In particular, starting 2011, the guaranteed operating temperature of the PCRAM device is made 125° C. for 10 years (see NPLT 3). If the phase-change material in the amorphous phase state is allowed to stand for a long period of time, sometimes the material will change to the crystal phase on its own. Due to this change, the data retention is impaired. Therefore, to achieve the above-mentioned guaranteed operating temperature, it is necessary to raise the crystallization temperature Tc of the phase-change material and to increase the activation energy when the amorphous phase crystallizes so as to raise the thermal stability of the amorphous phase. On the other hand, if the melting point of the phase-change material is high, a problem arises in that the energy (power) which is required for change from the crystal phase to the amorphous phase becomes larger, so a lower melting point is preferable.
PLT 1 discloses a nonvolatile memory which uses a GeSbTe compound as a phase-change material. However, the melting point Tm of the GeSbTe compound is a relatively high 620° C. or so, but despite this the crystallization temperature Tc is a relatively low one of about 160° C. or so (for example, PLT 2). Further, the activation energy of crystallization of the amorphous phase of a GeSbTe compound is about 1.5 to 2.2 eV (for example, NPLT 4), therefore the thermal stability of the amorphous phase is low and the data retention property can become fragile.
PLT 2 has as its object the provision of a phase-change substance layer which has a high crystallization temperature, has a low melting point, and is thermally and structurally stable and discloses a phase-change substance layer which comprises a four-component compound layer which includes indium, in particular, InaGebSbcTed (where, 15 (at %)≦a≦20 (at %), 10 (at %)≦b≦15 (at %), 20 (at %)≦c≦25 (at %), 40 (at %)≦d≦55 (at %)). That is, PLT 2 discloses a phase-change substance layer with a higher crystallization temperature than GeSbTe, but there is no description of the activation energy of crystallization which shows the thermal stability of the amorphous phase. Further, for measurement of the crystallization temperature, measurement of the reflectance is used. There is no description regarding the electrical resistance ratio between the amorphous phase and crystal phase. The thermal stability of the amorphous phase and data reading precision are unknown.
As a phase-change material which has a high crystallization temperature or has a high activation energy, PLT 3 discloses a phase-change material which comprises Sb and Te as main ingredients and at least one type of element added as additional elements. As additional elements, B, C, N, Ag, In, P, and Ge are described. That is, PLT 3 discloses a phase-change material which is comprised of Sb and Te as main ingredients and has at least one type of element added to it as additional elements wherein a 160° C. or higher crystallization temperature and a 2.5 eV or higher activation energy of crystallization is obtained. The examples of PLT 3 describe a phase-change material comprised of an Sb75Te25 alloy containing additional elements of N, Ge, B, P, and Ag. However, the phase-change material described in PLT 3 was invented as a phase-change recording material for an optical recording medium. There is no description regarding the electrical resistance ratio between the amorphous phase and crystal phase at all. Further, Te is a semiconductor, while the main ingredient Sb is a half metal, so the electrical resistance of the phase-change recording material is low. If used as a PCRAM memory device, there are the defects that the device resistance is low and the drive current for crystallization and amorphization easily becomes large (for example, see NPLT 5).
PLT 4 discloses a phase-change memory device which contains one or more elements which are selected from the group which comprises Te, Se, Ge, Sb, Bi, Pb, Sn, As, S, Si, P, O, and mixtures or alloys of the same. That is, PLT 4 describes a phase-change material where the ratio of ingredients of Te, Ge, and Sb is TeaGebSb100-(a+b) (a<70 (at %), b>5 (at %)), (TeaGebSb100-(a+b))cTMdSe100-c (a<70 (at %), b>5 (at %), 90 (at %)<c<99.99 (at %), and TM is one or more transition metals), and (TeaGebSb100-(a+b))cTMdSe100-(c+d) (a<70 (at %), b>5 (at %), 90 (at %)<c<99.5 (at %), 0.01 (at %)<d<10.0 (at %), and TM is one or more transition metals). However, there is no description regarding the crystallization temperature or the activation energy of crystallization of the amorphous phase and there is no description of the thermal stability of the phase-change material. Further, there is no description at all relating to the electrical resistance ratio between an amorphous phase and crystal phase.
Further, NPLT 6 describes a Ge—Bi—Te phase-change material. According to NPLT 6, it is described that in a phase-change material which is comprised of Ge—Bi—Te, a 240° C. or more crystallization temperature is obtained. However, there is no description relating to the activation energy of crystallization of the amorphous phase and thermal stability. Further, there is also no description relating to the electrical resistance ratio between an amorphous phase and crystal phase.
As explained above, among the already proposed phase-change materials, there is no material able to sufficiently withstand practical use which satisfies the requirements which are sought from materials of PCRAM memory devices such as 1) the high thermal stability of the amorphous phase and high data retention capability and, more preferably, 2) the low melting point and small energy (drive power) which is required for change from the crystal phase to the amorphous phase.